This invention relates to an avalanche photo diode provided with a hetero junction, and more particularly to an improvement in a guard ring structure provided for the purpose of preventing a breakdown at the peripheral portion of a photo detecting region.
Of various semiconductor photo detectors, an avalanche photo diode (hereinafter referred to simply as "APD") is a photo detector having characteristics of high sensitivity and wide band, and is widely employed in optical communications.
The APD performs amplification of a photocurrent by applying a reverse bias voltage to its pn junction to cause an avalanche multiplication under a high electric field. This develops a defect such that before the avalanche multiplication of the photocurrent occurs, electric fields center on the peripheral portion of the photo detecting region to cause a breakdown there.
To remove the abovesaid defect of the prior art, there has been proposed an APD in which a ring-shaped layer, commonly referred to as a guard ring, is provided to surround the photo detecting region, such as a single-element APD made of Ge or Si. However, a sufficient increase of the breakdown voltage cannot be attained by the provision of the conventional guard ring.